The field of the invention is measuring and defining tools and the invention relates more particularly to plumb lines of the type utilizing a light line with a weight (plumb bob) at the lower end thereof. Such bobs when suspended, of course, define a vertical.
In many building applications, it is necessary to indicate the vertical point in a confined area as within a wall such that it is not possible for the bob to about touch the floor. It is also often desirable to mark the vertical point below the plumb bob without disturbing the bob itself. By the use of a downwardly directed beam of light such purposes may be accomplished.
Lighted plumb bobs have been devised in the past with plumb lights being disclosed in U.S. Pat. No. 1,113,519 and in illuminated surveyor's instrument disclosed in U.S. Pat. No. 2,538,475. Plumb bobs having downwardly directed beams of light are disclosed in the U.S. Pat. Nos. 2,583,491 and 2,665,498. A major difficulty with such devices, however, was the ease and rapidity with which the batteries would become discharged when the plumb bob light was inadvertently left on. For instance, when a plumb light is being used in a confined, darkened environment, it is easy to see that the light is on, but when the bob is removed from this environment, the fact that it is on is not readily evident and thus the batteries are quickly dissipated and the device becomes of minimal use.